BENGALURU, Dec 15 (Reuters) - Digital payments firm Paytm"s (PAYT.NS)shares tumbled over 13% as a lock-in period for the company"s institutional investors ended on Wednesday, piling more pressure after a dismal debut last month. Paytm shares crashed more than 27% in India"s largest public offering last month. Since the listing on Nov. 22, the stock has logged losses for 13 of the 18 sessions. Early on Wednesday, the stock was trading at 1,269 rupees, compared to the offer price of 2,150 rupees. Paytm, which counts SoftBank (9984.T) and Ant Group among its backers, raised $2.5 billion in its IPO, of which $1.1 billion was from institutional investors. read more